Algebra-1

Question Analysis  
This question involves finding the common difference in a decreasing arithmetic sequence.  
The main focus is on recognizing that the common difference can be negative when the sequence decreases.  

Key Concept Explanation  
A negative common difference (d < 0) indicates that each term is smaller than the previous one, resulting in a decreasing sequence. The formula still applies, yielding a negative value in such cases.  

Step-by-Step Solution  
1. Take two consecutive terms, e.g., and .  
2. Calculate